1. Field of the Invention
The invention generally relates to control device and method of a vehicle including a lockup clutch capable of directly coupling the input and output of a hydraulic power transmitting device disposed in a power transmission path that extends from a power source, such as an engine, to driving wheels. In particular, the invention is concerned with switching control of the operating state of the lockup clutch.
2. Description of Related Art
There is known a control device of a vehicle including a lockup clutch capable of directly coupling the input and output of a hydraulic power transmitting device disposed in a power transmission path that extends from a power source, such as an engine, to driving wheels. The known control device is adapted to control the lockup clutch so that the lockup clutch is switched to an operating state that is determined from predetermined switching lines based on vehicle conditions.
An example of the control device of the vehicle as described above is disclosed in JP-A-9-280079. The vehicle disclosed in this publication includes a lockup clutch that directly couples the input and output elements, i.e., a pump impeller and a turbine wheel, of a torque converter that receives torque from the engine. The control device is adapted to switch the lockup clutch to an operating state that is determined based on vehicle conditions represented by the actual vehicle speed and the throttle opening, from a lockup region diagram having a boundary or boundaries of an engagement region and a release region of the lockup clutch, which boundary is determined in advance with respect to the vehicle speed and the throttle opening that represents an engine load as variables.
In the meantime, JP-A-2005-178626 discloses a vehicular integrated control system that controls a plurality of control systems for controlling a drive system, a braking system and a steering system, respectively, in an integrated manner. For example, the integrated control system harmonizes the required driving force calculated from the accelerator pedal travel or stroke as an amount of operation of the accelerator pedal by the driver, and the required driving force for controlling the driving force outputted from a driving assist system, such as a so-called cruise control system, for automatically controlling the vehicle speed irrespective of the accelerator pedal travel so as to achieve a target vehicle speed set by the driver. On the basis of the result of harmonization, the integrated control system produces command values to respective actuators for controlling the engine torque, change gear ratio of the transmission, and so forth.
When the integrated control as disclosed in JP-A-2005-178626 is performed, it is desirable to determine switching of the operating state of the lockup clutch based on the required driving force, so that control for switching the operating state of the lockup clutch is consistent with controls of the respective control systems. In this case, switching of the operating state of the lockup clutch may be determined from the above-mentioned lockup region diagram, based on the actual vehicle speed and the throttle opening into which the required driving force is converted, or may be determined based on the actual vehicle speed and the required driving force, from a lockup region diagram having the vehicle speed and the driving force as variables, into which the predetermined lockup region diagram having the vehicle speed and the throttle opening as variables is converted.
However, since the torque ratio t (i.e., turbine torque (output-side torque)/the pump torque (input-side torque)) of the torque converter changes depending upon the operating state of the lockup clutch, the required throttle opening (or the required engine torque) derived from the required driving force changes upon switching of the operating state of the lockup clutch even if the required driving force is constant. Namely, a vehicle condition based on which switching of the operating state of the lockup clutch is determined from the lockup region diagram changes with the operating state of the lockup clutch even if the required driving force is constant. As a result, a lockup ON determination (i.e., a determination that the lockup clutch should be switched ON) based on a lockup ON line used for determining switching of the lockup clutch from the released state to the engaged state and a lockup OFF determination (i.e., a determination that the lockup clutch should be switched OFF) based on a lockup OFF line used for determining switching from the engaged state to the released state are repeated in an alternating manner, thus giving rise to a possibility of switching control hunting.
In the first place, a situation where switching control hunting occurs will be specifically explained for the case where switching of the operating state of the lockup clutch is determined based on the actual vehicle speed and the required (target) throttle opening derived from the required (target) driving force, from a predetermined lockup region diagram having the vehicle speed and the throttle opening as variables.
FIG. 21 is a lockup region diagram in which a lockup ON (OFF→ON) line indicated by the solid line and a lockup OFF (ON→OFF) line indicated by the broken line are plotted in a two-dimensional coordinate system having the horizontal axis indicating the vehicle speed and the vertical axis indicating the throttle opening corresponding to the engine torque.
In FIG. 21, point A indicates vehicle conditions represented by a certain vehicle speed V1 and a required throttle opening θTHA that corresponds to required (target) engine torque and is derived from a required driving force F1 using the lockup-ON-time torque ratio (t=1) established when the lockup clutch is ON. Also, point B indicates vehicle conditions represented by the vehicle speed V1 and a required throttle opening θTHB that corresponds to required engine torque and is derived from the same required driving force F1 as that of point A, using the lockup-OFF-time torque ratio (for example, t=1.2) established when the lockup clutch is OFF.
When the lockup clutch is ON, the vehicle conditions represented by the vehicle speed V1 and the required driving force F1 pass the lockup OFF line and fall on point A, so that the lockup clutch is switched to the OFF state. If the lockup clutch is switched to the OFF state, the vehicle conditions represented by the same vehicle speed V1 and the same required driving force F1 pass the lockup ON line and fall on point B, so that the lockup clutch is switched to the ON state. In this manner, switching control hunting occurs depending upon the required driving force and the torque ratio.
In the next place, a situation where switching control hunting occurs will be specifically explained for the case where switching of the operating state of the lockup clutch is determined based on the actual vehicle speed and the required driving force, from a lockup region diagram having the vehicle speed and the driving force as variables, into which a predetermined lockup region diagram having the vehicle speed and the throttle opening as variables is converted.
FIG. 22 is a lockup region diagram in the form of a two-dimensional coordinate system having the horizontal axis indicating the vehicle speed and the vertical axis indicating the driving force of the vehicle. In FIG. 22, a lockup ON line AON indicated by the solid line and a lockup OFF line AOFF indicated by the broken line represent a pair of switching lines A for use in the lockup ON state in which the lockup-ON-time torque ratio (t=1) is used for converting the throttle opening into the driving force. Also, a lockup ON line BON indicated by the one-dot chain line and a lockup OFF line BOFF indicated by the two-dot chain line represent a pair of switching lines B for use in the lockup OFF state in which the lockup-OFF-time torque ratio (for example, t=1.2) is used for converting the throttle opening into the driving force.
In FIG. 22, point C indicates vehicle conditions represented by a required driving force F2 and a certain vehicle speed V2. If the vehicle conditions fall on point C when the lockup clutch is OFF, the switching lines B for use in the lockup OFF state are applied, and point C lies in the ON region with respect to the lockup ON line BON, so that the lockup clutch is switched to the ON state. If the lockup clutch is placed in the ON state, on the other hand, the switching lines A for use in the lockup ON state are applied, and point C representing the same vehicle conditions lies in the OFF region with respect to the lockup OFF line AOFF, so that the lockup clutch is switched to the OFF state. In this manner, switching control hunting occurs depending upon the required driving force and the torque ratio.